In the packet service (Global Packet Radio Service) in a GSM cellular network packets (logical link control or LLC frames) are exchanged between a mobile station MS and GPRS support nodes SGSN in the core network, both of which have the corresponding protocol layers. This exchange is handled by a processor called the packet control unit PCU, which is part of a conventional base station subsystem BSS.
Mobile stations communicating with the General Packet Radio Service (GPRS) of a GSM cellular network are assigned a mobile identity, called the Packet Temporary Mobile Subscriber Identity (P-TMSI). This P-TMSI is used on the GPRS Mobility Management layer (GMM). Any single mobile station can have several different types of P-TMSI, some of which are allocated by the network, for example by a GPRS Support Node (SGSN), during GPRS attach, Routing Area update or P-TMSI reallocation procedures. The GMM protocol layer also assigns a Tempory Logical Link Identity (TLLI) which is derived from the P-TMSI for use by the lower layers, such as the Logical Link control layer (LLC) and the Radio Link Control and Medium Access Control layers (RLC/MAC).
FIG. 1 illustrates the GPRS signalling plane for a mobile station MS, base station subsystem BSS and GPRS support node SGSN corresponding to GSM 03.60. At the top of the illustrated protocol stack both the mobile station MS and the GPRS support node SGSN have peer GPRS mobility management/session management GMM/SM protocol and logical link control protocol layers. The GMM/SM and LLC protocol messages are relayed by the base station subsystem, which does not have an equivalent protocol layer. On the Um interface between the mobile station and the base station subsystem Radio link control RLC and medium access control MAC protocol layers are present over the GSM radio frequency RF protocol layers. Across the Gb interface between the base station subsystem and the GPRS support node SGSN the base station subsystem GPRS protocol BSSGP is supported above the network service protocol and the layer 1 protocols.
This temporary logical link identity TLLI is one of the main addressing components for the exchange of packets between a mobile station and the core network in addition to a base station subsystem packet layer BSSGP virtual circuit identifier, referred to as the BVCI. Every packet between the mobile station and the core network will include the TLLI in a header.
While the packet control unit PCU uses the temporary logical link identity TLLI to assign packets to correct temporary block flows TBF and to direct packets to the corresponding mobile station, the packet control unit PCU has no previous knowledge of this identifier. The mobile station includes its assigned TLLI in uplink messages. The core network also transmits the TLLI if it has data to send but the mobile station is in idle mode, i.e. no TBF exists.
When LLC frames are sent from a mobile station to the core network the packet control unit PCU identifies the temporary logical link identifier TLLI when an uplink temporary block flow (TBF) is established.
For LLC frames sent to the mobile station, the packet control unit PCU directs the LLC frames to one particular cell (BVC), and uses the established downlink temporary block flow (TBF) if this is available. If no downlink temporary block flow is available the packet control unit PCU can assign the mobile station to a downlink temporary block flow using the temporary logical link identifier TLLI.
The TLLI is essentially a temporary telephone number that is used when the mobile station is in a specific routing area. A TLLI is valid for as long as a mobile station remains in a Routing Area, which comprises several cells. The GMM protocol layer in the mobile station informs the network of the location of the mobile station.
When a mobile station is not active in a packet transfer, the GMM protocol layer is in a standby state, called GMM-STANDBY. In this state, the GMM protocol layer of the mobile station notifies the core network when the mobile station crosses from one routing area into another with a Routing Area Update. Any messages that need to be sent by the core network to the mobile station while the mobile is in this state will require that the mobile station be paged in all cells belonging to its current Routing Area. When the mobile station sends an uplink data packet (LLC-PDU) to the cores network, it enters the GMM-READY state. When the mobile station is in GMM-READY state, i.e. is active in packet mode transfer, the GMM layer notifies the network whenever a cell border is crossed, by performing cell updates. In this state, the exact location of the mobile station is known on cell level; any packets transmitted to this mobile station by the core network can thus be sent directly to the mobile station via the correct base station subsystem (BSS). A mobile station remains in the GMM-READY state for a set period of time after sending an uplink LLC packet. A timer, called the READY-STATE TIMER is used to count down this period and is reset with each uplink LLC packet sent. The cell update message does not reset the timer. The timer value is coordinated between the mobile station and the GPRS support node SGSN during the GPRS attach and Routing Area Update procedures. The GMM states thus exist in both the mobile station and the GPRS support node SGSN to which it is connected.
Recently proposals have been made to extend conventional cellular networks by including access networks that utilise a low power unlicensed-radio interface to communicate with mobile stations. The access networks are designed to be used together with the core elements of a standard public mobile network and consist essentially of plug-in low-power unlicensed radio transceivers, or access points, each designed to establish an unlicensed radio link with a mobile station MS and a controller or interface node connecting the unlicensed radio transceivers with the mobile core network. The connection between the controller and the access points is a fixed broadband network. Suitable unlicensed-radio formats include digital enhanced cordless telecommunications (DECT), wireless LAN and Bluetooth. An adapted mobile handset capable of operating over both the standard air interface (e.g. the Um interface) and the unlicensed-radio interface means that the subscriber requires only one mobile station for all environments. The access network should be constructed in such a way that the core elements of the public mobile network view the interface node as a conventional base station controller BSC. In other words, the interface between the access network and the core network follows 3GPP specifications.
However, the combination of the small size of the access point coverage areas and the ease with which these can be installed and moved means that each access controller will be controlling a very large but frequently changing number of cells compared to the equivalent base station controller of a conventional cellular network. Moreover, mobile stations served by an unlicensed-radio access network are connected to the access network by at least one dedicated control connection, typically a static TCP connection over the broadband network connecting the access controller to the access points. The conventional mechanisms for routing packet service data between the mobile station and the core network are thus not useful in this access network for reasons of scale and signalling load.